The invention relates to methods and devices for manufacturing blown films.
In conventional methods for manufacturing blown films, relatively major fluctuations in film thickness occur which can be as great as 20% depending on the film thickness and the quality of the manufacturing installation. These fluctuations in thickness have a variety of causes, for example the inhomogeneity of the melt, temperature differentials in the melt and hence in the tool, and mechanical defects and adjustment errors of the tool and the cooling system. It is known that the film thickness can be controlled by changing the flowrate of the melt within the tool by controlled heating or cooling of the tool in certain circumferential areas. However, the corresponding devices and tools are very expensive, and a regulating system operating in accordance with this method for correction of film thickness is relatively slow, since long delays occur during the heating and cooling of the tool areas.
German Patent 3,627,129 teaches a method of the type recited at the outset in which the film thickness is influenced by controlling the cooling air stream. This method makes use of the fact that hotter areas of the film bubble in the cooling zone between the nozzle and the frost line undergo greater stretching because of the lower viscosity of the melt than do the cooler areas, so that the thickness of the film can be increased by more intensive cooling and reduced by less cooling. To control the cooling air stream, a plurality of pins is distributed on the circumference of the cooling ring near the air outlet, and the flow resistance in the individual circumferential areas of the cooling ring is varied, with the pins, which act as interfering elements in the cooling air flow being extended to a greater or lesser degree. The effect of such interfering elements on the cooling air flow is described in Plesske: "Blown Film Cooling: Developmental Status and Effect of Defects on Film Quality," in Kunststoffe, Vol. 69, No. 4, pages 208 to 214, 1979.
As the cooling air flows around the interfering elements, it is vorticized behind the interfering elements and uncontrollable local fluctuations in the cooling air flow are generated at the die gap, so that a uniform film thickness can be obtained only with difficulty.
Another problem consists in the fact that the increase in flow resistance produced by the interfering elements results in an increased backpressure upstream of the interfering elements at one point on the circumference, so that the air throughput is increased in the adjacent circumferential areas. Hence, a complicated system of interactions exists between the cooling air throughputs in the various circumferential areas which is difficult to control by regulation. Because of this problem, it is difficult to control interfering elements on the basis of areas in such fashion that the thickness profile of the film is regulated in a closed regulating circuit.
DE-OS 36 23 548 teaches a method in which the cooling air is supplied through two annular die gaps. The two die gaps are provided with separate cooling air supply devices, so that the air throughputs through the two die gaps can be controlled independently of one another. In the device proposed for working this method has an essentially uniform flow profile in the circumferential direction of the die gaps, however.
DE-A-26 58 518 describes a method with the general features of the invention. In this method, the thickness profile of the film is regulated with the aid of a crown of correcting air nozzles, in which the cooling air throughput is controllable with the aid of a valve for each. However, since the individual correcting air nozzles are supplied by a common annular line, adjustment of a single valve results in a change in the pressure distribution in the annular line and hence to undesired feedback effects on the throughput through the adjacent correcting air nozzles. This makes regulation of the complete system difficult.
U.S. Pat. No. 4,443,400 teaches a device by which a greater film thickness can be set in certain circumferential areas of the film. For this purpose, additional air nozzles are provided in the circumferential areas in question, each of said nozzles being connected by a common valve with the annular chamber of the cooling ring. If the valve is opened further, the total flow resistance in the circumferential area in question decreases, and a greater total volume of cooling air is discharged through the die gap of the cooling ring and through the additional nozzles.
U.S. Pat. No. 4,209,475 teaches a device by which the thickness profile can be controlled in such fashion that the width of the die gap of the cooling ring can be narrowed segmentwise. Here again however, undesirable interactions between adjacent segments occur, since the narrowing of the gap in one segment results in an increased cooling air throughput in the adjacent segments.
The goal of the invention is to control the cooling air flow in such fashion that an independent, sensitive, and rapid modification of the cooling effect is possible in the individual circumferential areas of the film bubble and extreme local fluctuations in the cooling effect are avoided.
Solutions according to the invention are described in the independent claims.
According to the basic idea of the invention, at least one cooling ring divided into individual circumferential segments is provided, and the cooling air streams in the individual segments are controlled or regulated independently of one another. In the proposed solutions, assurance is provided by supplementary measures that the cooling air throughputs in the individual segments do not influence one another and that no abrupt changes in the cooling effects occur over the circumference of the cooling ring.
The principle of the solution is that a main cooling air stream is generated with the aid of a conventional cooling ring, in which stream the flowrate of the air over the entire circumference of the film bubble is as uniform as possible and that deliberate local modifications in cooling effect are achieved by either supplying additional cooling air through a separate air gap or drawing off a portion of the main cooling air. The highly efficient and correspondingly slow cooling alr blower to generate the main cooling air stream can therefore be operated at a constant backpressure while the flows in the individual circumferential segments of the additional air gap can be varied rapidly because of the low throughput, so that delicate control of the circumferential distribution of the total cooling air flow becomes possible. In addition, the uniform main cooling air stream erases the differences in throughput between the individual circumferential sections of the additional air gap to a certain extent, so that excessive disturbances of the total cooling air stream and abrupt local changes in cooling effect are avoided.
Since the method according to the invention requires no special design of the cooling ring that serves to generate the main cooling air stream, the method can be worked by suitable retrofitting, even in existing blown film machinery.
The method also has the advantage that the addition of additional cooling air improves the cooling effect overall and allows a corresponding increase in the efficiency of the system. Moreover, an increase in cooling efficiency is achieved by the fact tilat the air flow is more strongly vorticized by blowing the additional cooling air into the main cooling air stream. The initially turbulent upwardly directed main cooling air stream, because of the decreasing flowrate, normally changes to a laminar flow as it moves upward, so that the cooling effect decreases sharply upward. The addition of the additional cooling air at a suitable level can generate new turbulence so that the effective cooling area is increased. Changing the cooling effect is then accomplished without a considerable change in the flowrate and pressure, so that a stable bubble position can be ensured.
The increase in cooling effect can be controlled not only by modifying the throughput of the cooling air supplied through the additional air gap but alternatively, or in addition, by segmental changing of the angle of incidence of the additional cooling air. Optionally, the positions of the individual segments relative to the film bubble in the vertical or radial direction can be varied.
A further increase in the sensitivity during control of the circumferential distribution of the cooling effect can be achieved by supplying precooled air through the additional air gap. In this case, the circumferential distribution of the cooling effect can also be controlled by segmental modification of the temperature of the additional cooling air. This solution can be implemented structurally, for example, by supplying cooled and uncooled different amounts to the individual segments of the additional air gap through mixing valves or by spraying a coolant into the additional air flow.
Optionally, instead of using air, other cooling gases with different heat capacities may be used so that the circumferential distribution of the cooling effect can also be controlled by the composition of the gas mixture.
An important advantage of the solutions described above also consists in the fact that, because of the separate supply of cooling air or cooling gas to the individual segments of the additional air gap and because of the separate exhaustion of the air in the individual segments, influence of the additional cooling air streams on one another is avoided. This permits stable regulation of the thickness profile in a closed regulating circuit.
In another solution, a portion of the cooling air stream is separated at positions distributed circumferentially on the cooling ring, at the die gap or upstream thereof, and the quantity of cooling air thus deflected is controlled by adjustable guide blades or guides.
By separating a portion of the cooling air, the throughput at the die gap can be controlled with precision and a high backpressure can be prevented from developing upstream of the divergence point and the cooling air escapes through adjacent circumferential areas. Changing the position of the guide blades in one circumferential area hence has no effect on throughput in the other circumferential areas. In addition, diverting the cooling air prevents the flowrate from increasing as it flows around the guide blades and produces a strong vorticization downstream from the guide blades. Adjusting the guide blades therefore permits simple, accurate control of the circumferential distribution of the cooling air throughput and hence the thickness profile of the film bubble.
In one preferred embodiment of the device, the upper wall of the cooling ring is surrounded by a crown of exit openings for the diverted cooling air, and each individual exit opening has a guide blade associated therewith which projects from above into the interior of the cooling ring and deflects a portion of the cooling air into the exit opening. The positions of the guide blades can be adjusted vertically so that the volume of diverted cooling air can be varied.
It is possible in this embodiment to divide the cooling ring into individual segments using radial partitions so that the cooling air streams are separated from one another before they reach the die gap or a position a short distance upstream from the die gap. In this manner, the throughput differences between the individual segments downstream from the guide blades are prevented from equalizing once more and the exit openings and guide blades can be located relatively far out on the cooling ring leaving more space for the adjusting mechanisms.
In another embodiment, the guide blades are formed by a lip of flexible material which is continuous in the circumferential direction, the angle of incidence of said lip being adjustable in the individual circumferential areas with the aid of rams or the like. This design provides structural simplification and it is possible to make the shape of the lip streamlined in such a manner that vorticization of the cooling air downstream from the lip is avoided. In addition, in this design discrete transitions in the circumferential distribution of cooling air flow are avoided.
Adjustment of the guide blades and/or rams can be accomplished manually using adjusting screws or the like or with the aid of suitable drives, for example electromagnetic, pneumatic, or piezoelectric adjusting elements. In the latter case it is possible to regulate the cooling air throughput in the individual circumferential areas on the basis of the film thickness measured at various points on the circumference of the film bubble. Since the changes undertaken within the scope of regulation affect the settings of the guide blades only in the affected circumferential area and have no significant feedback on the other circumferential areas, the settings of the regulating system, in other words the positions of the guide blades, are largely decoupled, so that the tendency of the regulating system to oscillate is reduced and stable regulation is made possible.
To reduce even further the influence of the cooling air streams on one another, it can be advantageous to control the flow resistances for the air streams diverted through the outlet openings in such fashion that they correspond to the flow resistance of the die gap of the cooling ring for every position of the guide blades. In this manner, the flow and pressure conditions in the distribution chamber at the outer circumference of the cooling ring can remain practically completely unaffected by the adjusting movements of the guide blades. Adjustment of the flow resistance can be accomplished with the aid of electromagnetically controlled metering valves and the like. However, it is also optionally possible to couple the guide blades mechanically with a throttle part that narrows the corresponding outlet opening to a greater or lesser degree depending on the position of the guide blades.
The cooling effect in the individual segments of the cooling ring can also be varied in such fashion that the flowrate of the cooling air is controlled by narrowing or expanding the die gap. For a given throughput, increasing the flowrate produces an increase in the cooling action. This principle forms the basis of another embodiment. A nearly constant throughput is achieved by virtue of the fact that the change in flow resistance caused by the change in the gap width is compensated by a greater or lesser throttling of the flow upstream of the die gap. This solution has the advantage that the total cooling air throughput in each circumferential segment remains constant. In this manner, a local reduction of the aftercooling of the film bubble above the frost line is prevented and assurance is provided that the film temperature will have decreased everywhere when the film is flattened and wound up so that the film layers will not stick to one another. Because of the uniformly high aftercooling effect, the length of the aftercooling section can be reduced and/or film ejection can be increased.
When working with one of the two solutions described above with a die gap that is divided into individual segments, the disturbances in the cooling air flow produced by the partitions can have different effects on the cooling capacity. As tests have shown, in the case of a die gap which is located close to the film bubble, the vortices produced by the partitions result in intensification of the cooling effect and hence thickening of the film. If on the other hand the die gap is located further from the film bubble, the influences of the different flowrates of the laminar exit flow will predominate. With a uniform width of the die gap, a squared velocity profile is obtained in the circumferential direction, so that the speed in the vicinity of the partitions is less than in the middle of each individual segment. In this case, therefore, there is a reduced cooling effect in the vicinity of the partitions and a thinning of the film. Other embodiments propose various measures for avoiding these disturbing effects.
Preferred embodiments of the invention will now be described in greater detail with reference to the drawings.